The isotope effect on the internal mobility of the dysprosium ion in molten dysprosium chloride has been measured from 973 to 1073 K by countercurrent electromigration. Heavier Dy isotopes have been enriched toward the anode. The mass effect defined by mu = (Delta u/u(Dy))/(Delta M/M(Dy)) has been calculated to be -0.032 +/- 0.002, where hu is the difference in the mobilities of M (mass number) = 160, 161, 163, or 164 with reference to that of M = 162. A comparison of this value with those previously determined for other molten metal chloride systems suggests that the main electrically conducting species containing Dy is the trivalent cation and not the species shown to exist in the melt by other techniques.